Memory modules such as single in-line memory modules (SIMMs) and dual in-line memory modules (DIMMs) are typically used to store data, instructions, and other information in computers or other digital systems. FIG. 1 shows a typical memory system in which processor 102 communicates with dynamic random access memory (DRAM) devices 110–117 on memory modules 106 and 108, respectively, via memory controller 104. Memory controller 104 communicates appropriate memory instructions (e.g., write, read, refresh, etc.) to memory module 106 via address and command bus 118, and to memory module 108 via address and command bus 120. Data is transferred from memory controller 104 to both modules via bus 122.
There are at least two disadvantages associated with the conventional memory system of FIG. 1: (1) buses 118, 120, and 122 are multi-drop buses that have high capacitive loads which require large drivers in memory controller 104 and in DRAMs 110–117 (to drive bus 122); and (2) there tends to be large mismatches in loading between the address and command buses and data bus 122. These disadvantages combine to reduce the maximum operating frequency, increase power consumption, and decrease performance of the system. Additionally, memory controller 104 may include a high number of pins (e.g., approximately 190 pins for a 4 DIMM SDRAM system) to support access to memory modules 106 and 108. The bus width of data bus 122 is often 64 or 72 bits wide to support larger bandwidths, for example, of up to 100 mega transmissions per second (MT/s).
FIG. 2 shows another typical memory system that uses technology from Rambus, Inc. of Mountain View, Calif. In this system, processor 202 communicates with Rambus dynamic random access memory (RDRAM™) devices 210–217 on memory modules 206 and 208 (also called Rambus in-line memory modules or RIMM™ modules), respectively, via memory controller 204. Memory controller 204 communicates appropriate memory instructions (e.g., write, read, refresh, etc.) and data in a time-multiplexed fashion to memory modules 206 and 208 via memory specific bus 226. Each of the RDRAM™ devices 210–217 include interface logic 218–225, respectively, that enables the RDRAM™ devices to interface to memory specific bus 226. Memory specific bus 226 may operate at a high frequency (e.g., 250 to 400 MHz), have a small bus width (e.g., 16 or 18 signal lines), have symmetric loading for address, command, and data information, and have a bandwidth of up to 800 MT/s. In other embodiments, the address and control information may be separated (demultiplexed) from the data bus in this environment.
The system of FIG. 2 has a number of disadvantages. Each RDRAM™ device includes a significant amount of logic in the interface logic which makes implementation of the RDRAM™ devices more difficult and more expensive than other DRAM devices, and which causes the RDRAM™ devices to draw more power and dissipate more heat than if the logic was not present. This may lead to thermal and reliability problems. Additionally, each RDRAM™ typically includes a delay locked loop (DLL) circuit coupled to a plurality of clock signals on bus 226. The DLL circuits are typically always functioning and drawing significant amounts of power which contributes to thermal and reliability issues. Additionally, memory controller 204 must include relatively large drivers to drive bus 226.
In each of the memory systems described in FIGS. 1 and 2, the memory controller must be designed to provide the appropriate memory transactions to the memory devices in a predetermined format and at predetermined times. The memory devices and memory controller must then be designed to work together. It would be advantageous to decouple the design of the memory devices and the memory controller such that independent advances may be made in each technology. It would also be desirable to decouple the operation of the memory devices from the memory controller to decrease power drawn by each memory device and by the entire memory system.
Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows.